A single day in March foreshadowed news this week that, by one tracker’s measurements, Google Chrome has leapfrogged Microsoft Internet Explorer to become the No. 1 Web browser.

That’s the word from Ireland-based StatCounter, whose figures Microsoft has disputed in the past. To be sure, other trackers still give IE a solid lead.

In fact, it appears that Chrome’s lead again was for a single day, though the trend favors Google. StatCounter had Chrome edging IE on Sunday. Figures for Monday and Tuesday weren’t immediately available.

Still, by passing Microsoft‘s (MSFT) Internet Explorer browser for at least two days in the past three months, Chrome did what its older rivals — Apple‘s (AAPL) Safari and Mozilla’s Firefox, among others — had never been able to do. It unseated the king that’s been atop of the field since overtaking pioneer Netscape in the 1990s.

And as Chrome becomes more popular, observers say the 4-year-old browser seems poised to be a sort of envelope product connecting Google’s Gmail, YouTube, Google+ and other products and helping Google better track users over multiple services.

“More than anything, I think people are sick of Microsoft and their lack of innovation,” he said. “They’re thinking about using different software.”

While IE has introduced nine versions in 17 years, Chrome has launched 18 versions in less than four years.

Chrome is one of the “strategic growth areas” and long-term projects “that were seen as crazy when we launched them but now have phenomenal usage,” Google co-founder and CEO Larry Page said in an April conference call with analysts to discuss Q1 financial results.

Chrome’s been downloaded more than 200 million times, Page says. The company last quarter launched a version for its Android, the most widely used mobile operating system.

The metric that showed Google surpassing IE that day in March can be misleading, because it was on a weekend, StatCounter CEO Aodhan Cullen said in a statement, before Chrome again beat IE this past Sunday. He points out that IE is the default browser for many companies, and is on far more computers than Chrome. Microsoft says it has 48% of the U.S. market for Windows-based computers vs. Chrome’s 15%.

StatCounter’s daily figures in the past couple months give IE weekday leads over Chrome of five, six, seven or more percentage points. The weekends have been very close, though. On Saturday, IE just edged Chrome by .02 percentage point. The figures aim to track global usage by overall browser market share.